Method of jacketing a fissionable body



Feb. 16, 1960 E. C. CREUTZ METHOD OF JACKETING A FISSIONABLE BODY FiledMay 22, 1945 'aug @e @E l United States Patent METHUD F JACKETING AFISSIONABLE BODY Edward C. Creutz, Santa Fe, N. Mex., assignor to vtheUnited States of America as represented by the United States AtomicEnergy Commission Application May 22, 1945, Serial No. 595,182

6 Claims. (Cl. 29-473-3) This invention relates to neutronic reactorsand to a novel article of manufacture used in and in combination withsuch reactors. In neutronic reactors a neutron iissionable isotope suchas U233, U2, or 94239 or a mixture thereof is subjected to fission byabsorption of neutrons and a self-sustaining chain reaction' isestablished by the neutrons evolved by the fission.

In general such reactors comprise bodies of compositions containing suchfssionable material, for example, natural uranium, disposed in a neutronslowing material which slow the neutrons to thermal energies. Such aslowing material is termed a neutron moderator. Carbon, beryllium, andD20 are typical moderators suitable for such use. Heat is evolved duringthe reaction which is removed by passage of a coolant through thereactor or in heat exchange relationship therewith. Specilc details ofthe theory and essential characteristics of such reactors are set forthin co-pending application of Enrico Fermi and Leo Szilard, Serial No.568,904, tiled December 19, 1944, now Patent No. 2,708,656, issued May17, 1955.

In the operation of neutronic reactors at a substantial power, forexample, 1000 kilowatts or more, for more than a few minutes difficultycan be encountered due to the fact that the moderator and/or coolant canbecome highly radioactive due to the projection of fission fragmentsand/or recoils into the surrounding material during the operation of thereactor if the fissionable material is bare. This result can seriouslycomplicate operation of the reactor since the radioactivity of thecoolant and moderator may constitute a serious hazard to personnel ifnot completely shielded.

Improved neutronic reactors have been produced wherein the tendency ofthe moderator and/or coolant to become radioactive is substantiallyminimized. It has been found that this result may be secured byinterposing a solid non-iissioning barrier between the iissioningisotope and the coolant or moderator or both. Notwithstanding the factthat introduction of non-fissioning isotopes into a neutronic reactorabsorbs neutrons and, therefore, may tend to prevent establishment of aneutron chain reaction, it has been discovered that it is possible toinsert non-iissioning barriers into the reactor, and to so proportionthe thickness of the barrier to prevent passage of all or a preponderantamount of iission fragments and/or recoils from the iissioning isotopeinto the moderator and/or coolant while yet avoiding use of such aquantity of barrier that thev neutron absorption thereof preventsestablishment or maintenance of the neutron chain reaction. The barrieritself should be constructed of a material or materials having a lowneutron capture cross-section.

In accordance with the present invention it has been found that steelparticularly stainless steel which contains corrosion resistant elementssuch as nickel, chromium and/or cobalt, while having a comparativelyhigh neutron capture cross-section may be used as a barrier or sheathfor tissionable bodies in a reactor and has '2 the advantage ofwithstanding high operating temperatures when the barrier is in contactwith a coolant.

In order to promote heat transfer from the ssioning isotope through thebarrier to the contacting moderator and/ or coolant, the barrier shouldbe in close intimate contact with lissioning composition. Generally,special precautions are observed for this purpose. Thus, a shell formingthe non-iissioning barrier or sheath may be drawn or shrunk upon thessioning component which preferably is in the metallic state. Therefore,the barrier material must be capable of being shrunk or drawn to a verysubstantial degree so that considerable reduction in the diameter of thesheath or barrier may be effected after the sheath has been fitted uponthe vssionable body. It is possible to reduce stainless steel by atleast 30 percent and thus obtain a very tight it on the uranium or otheriissionable material. These enclosed iissionable components are used asthe active component of a neutronic reactor, and frequently are locatedin contact with a coolant, such as a gas or liquid coolant, for removalof heat from the enclosed ssionable components and from the reactor.Where the coolant has a corrosive effect upon the iissionable metal orother material, the non-fissioning sheath also serves to protect thessionable material. In such a case, every precaution, including choiceof barrier material, must be taken to prevent the existence oroccurrence, following the drawing operation, of pores in the sheaththrough which the coolant may enter. Stainless steel is suitable foreliminating such pores even in very thin sheaths. Frequently the sheathsare hermetically sealed about the fissionable material in order to sealthe material from the coolant. The problem of welding or otherwisesealing the sheath or barrier about the issionable material presentsdifficulties which are readily overcome by the use of stainless steel asthe sheathing material.

As previously noted, lengths of the ssionable bodies generally aredisposed in channels extending through or at least into the moderator.During the chain reaction the central interior portion of these bodiesfrequently rises to a temperature much above the temperature of the bodyexterior which is in close relationship with the coolant. The barriermaterial must, therefore, be capable of withstanding the temperature ofcontact with the lissionable component without deteriorating.

The provision of a new article composing a fissionable component and anenclosure or barrier therefore which is capable of surmounting theforegoing diiculties and problems constitutes the principal objects andadvantages of the invention, others of which will become apparent fromthe following description anddrawingsin which:

Fig. l is a longitudinal sectional View of a ssionable body looselyenclosed in a barrier or sheath of steel;

Fig. 2 illustrates the assembly of the steel jacket or barrier as it isdrawn through a die into tight contact with the issionable body; and

Fig. 3 is a longitudinal view of the completely sealed, jacketed body.

As illustrated in the drawings, the invention generally comprises aprotective coating or jacket 2 of stainless steel completely enclosingand sealing a uranium rod 1.

Inasmuch as the stainless steel protective coating or jacket has theproperty of absorbing neutrons to some extent, it is desirable tomaintain the quantity of the steel within the pile at a minimum, inorder to make possible a chain reaction therein as previously discussed.Consequently, the thickness of the steel tubes and jackets should bemaintained 'at the minimum required from the standpoint of strength,possible corrosion and erosion, and ability to restrain ssion productsto a substantial degree. Where the jacketed bodies are exposed directlyto the attack of a corrosive coolant, such as water, the

Patented Feb. 16, 1960 tubes generally is of theorder of 0.005 to 0.035inch.

For convenience in handling the uranium rods or tubes,

they generally are made in relatively short lengths, for

instance, about 8 inches in length. r-Several methods may be employedfor hermetically sealing the uranium body 1 in the thin jacket 2.ofstainless steel or similar material.

A disc `6 of stainless steel serves as a closure, and is tightly weldedto the open end ofthe jacket. If' desired, disc 6 may be provided with arim or other-projectionA tl projecting outwardly from the outer surfaceof the disc, and concentrically contiguousto the open end ofthe jacket2. Sealing closed the open end of the jacket 2 is accomplished byywelding together the yjacket Zand the cap rim 8 as at 9.

The assemblage may beV constructed in the following manner. The uraniumrod 1 of proper dimensions is shot blasted, cleaned and polished.Thereafter, the rod 1 and cap 6 are introduced into the can or jacketZ,and the can is placed in a tightly fitting steel sleeve or `die 3.Thereupon, the jacketed uranium .rod and cap are forced by hydraulicpressure exerted .upon piston 10 through the die usually atan elevatedtemperature. A secondl pass through amore restricting die maybedesirable to insure tight contact of the jacket with the rod. Theassemblage is then chilled; the edges ofthe cap rim 8 and the jacket arewelded together; and the extending edges turnedinwardly .into contactwith the end cap 6. A second or outer stainless steel cap 12 isthereafter Welded to the jacket 2 exterior of the inner cap or disc 6.

The jacketed bodies after production may be disposed in the channels `ofa neutronic reactor in end to end relationship. The usual structure of acarbon moderated reactor comprises a carbon cube with horizontalchannels in which the jacketed ssionable bodies -lie and through whichthe coolant passes. The jacketed bodies herein described may be usedeffectively in this reactor or any other type of reactor. Fissionablebodies other than natun l. A method of jacketing a ssionable body whichcomprises tting the body intoa steel jacket,` inserting a steel rimmedclosure disc into the jacket, said rim projecting outwardly from theouter surface of the disc, drawing said jacket through a die into closecontact with said body, and welding the rim to the jacket to formanimpervious seal therebetween.

2- Amethpd of iaeketing a ssidnable body which eemprises tting the bodyinto a steel jacket, inserting a steel `rimmed disc into the jacket,drawing said jacket through a die inte close veentaet with seid body,welding the rimmed disc to vsaid jacket forming an impervious sealtherebetween, and turning said jacket and the rim of said disc inwardlytooverlie said disc.

3. The method ofjacketingan Yelongated body of material which comprisesinserting the body into a cupshaped jacket, inserting an outwardlyfacing cup-shaped closure member into the open end of the jacket,drawing the jacket'into close contact with the body and with the closuremember, welding-the wall'of the closure member to the `wall of thejacket, and bending the welded portion inwardly to form AaAsubstantially flat end.

`4. lThe method of claim 3 wherein the body has a shoulder portionadjacent the open end of the jacket, and the closure member `has bothinwardly and outwardly facing cup portions, the inwardly facing cupportion fitting'tightly over saidshoulder portion.

5. A method of jacketing a body which comprises fitting the bodyinto ametal jacket, inserting a'metalrimmed closure disc into Vthe jacket,said rim .projecting outwardly from the outer surface of the disc,drawing said jacket through a die into close contact With said body, andwelding the rim to the jacket to form an impervious seal therebetween.

6. A method of jacketing a body which comprises litting the `body Vintoa lmetal jacket, `inserting a rimmed metal disc vinto the jacket,drawing said jacket through a die into close contact vwith said body,welding the rimmed disc to said jacket forming an impervious Sealtherebetween, and -turning said jacket and the rim of said disc inwardlyto overlie said'disc.

References Citedinthe die .0. f this patent UNITED STATES PATENTS

1. A METHOD OF JACKETING A FISSIONABLE BODY WHICH COMPRISES FITTING THEBODY INTO A STEEL JACKET, INSERTING A STEEL RIMMED CLOSURE DISC INTO THEJACKET, SAID RIM PROJECTING OUTWARDLY FROM THE OUTER SURFACER OF THEDISC, DRAWING SAID JACKET THROUGH A DIE INTO CLOSE CONTACT WITH SAIDBODY, AND WELDING THE RIM TO THE JACKET TO FORM AN IMPERVIOUS SEALTHEREBETWEEN.